1. Field of the Invention
The invention relates to a system for storing electrical energy which may be either exteriorly supplied or generated in situ by a photoconversion electrode. The invention relates to one particularly preferred type of storage system, as well as to various structural embodiments for batteries including storage systems and to methods of constructing said systems.
2. Description of Prior Art
Numerous systems for the storage of electrical energy are described in textbooks and in the patent literature. Most such systems are based on two solid electrodes in a liquid electrolyte with the electrodes undergoing redox reactions essentially in the solid state. In a newly-developed system, the redox cell comprises two liquid redox systems separated by a membrane. In this system, the redox reactions occur in the liquid at metal or carbon electrodes. Also, hybrid systems have been developed recently, where two liquid systems are separated by a membrane, and in one compartment the redox reaction occurs by the plating out of a metal. The best known examples of such systems are the zinc/bromine and zinc/chlorine batteries.
U.S. Pat. No. 4,064,326 describes an in situ storage system as it is used in photoelectrochemical cells. This patent discloses various storage systems, including a system in which the storage electrode is formed of solid tin. In this system, the storage compartment includes an alkaline sulfide solution. The electrochemical reaction disclosed in the patent is: EQU Sn.sub.(s) +S.sup.- .fwdarw.SnS.sub.(s) +2e.sup.-
Thus, in such a system, during discharge, solid tin is disclosed as being converted to solid tin sulfide with the release of electrons. The reaction as set forth above appears to suggest a redox system in which both the reactant tin and product tin sulfide are present in the solid phase with the reaction occurring at the electrode itself. While the patent discloses the use of a membrane generally for separating the storage compartment from the rest of the cell, the use of a membrane is clearly not always necessary and is not suggested in such a system. Furthermore, when used in conjunction with a photoconversion system having a polysulfide electrolyte, one would not normally have considered a membrane to be necessary by virtue of the "solid state" reaction which was thought to occur.
Similar systems have been described in U.S. Pat No. 4,128,704. In Nature, Vol. 261, No. 5559, pp. 403-404 (1976), the Ag/Ag.sub.2 S system was described. The Journal of the Electrochemical Society, Vol. 124, 532 (1977) discloses Sn/stannate and Zn/zincate systems. Bipolar electrodes have been described in Proc. Electrochem. Soc., Vol. 73-3, page 34 (1977) and in U.S. Pat. No. 4,315,973, the disclosure of which is incorporated herein.
Many membrane systems suffer from the fact that very high demands are placed on the selectivity of the membrane, because mixing of the redox systems leads to deterioration of the storage system. Other systems which do not suffer from this disadvantage tend not to be compatible with stable photoelectrochemical electricity generators, and therefore are not suitable for in situ generation of photoelectrochemically generated electrical energy. No satisfactory system has been described where nonselectivity of the membrane can be tolerated, which is suitable for in situ generation in a photoelectrochemical conversion system, and which lends itself easily to bipolar connections.